In 3D radiation imaging (see FIG. 1), such as computed x-ray tomography or 3D interventional x-ray imaging, projection images of a subject are acquired from different viewpoints and volumetric image data is reconstructed out of the acquired projections. If a spatially isotropic resolution is desired the angular range of the information content in the projection images has to span 180° plus a fan angle of the radiation beam. In most, if not all, current 3D radiation imaging systems a certain field of view of the subject is illuminated by an x-ray beam spanning a certain fan angle. To reduce radiation received by a subject the beam is usually filtered by a radiation filter, usually a homogeneous radiation absorbing solid or liquid, but also, less common, a homogeneous gas-filled x-ray filter as disclosed in DE 102 27 808 A1. Because attenuation of the x-ray beam is strongly dependent on a local thickness of a subject to be imaged (such as a patient body), thinner parts (e.g. edges) of the subject are over illuminated with respect to thicker parts. Furthermore typical object or patient shapes are not cylindrical so the optimal collimation of the x-ray beam depends on an angular direction of each projection.
To overcome this problem beam shapers are known. These are x-ray filtering devices that are able to shape the x-ray beam depending on the angular direction of each projection. The most common beam shaper is a so-called bow-tie filter, called as such due to its shape. Bow-tie filters attenuate more x-rays at the outside and less on the inside of the filter, thereby reducing x-ray dose on the edges of the subject. However, such beam shapers have certain shortcomings for use in 3D imaging systems. This is illustrated with the help of FIGS. 2a and 2b. 
FIG. 2a zooms in on an active area of a 3D imager 10, 10′. The subject to be imaged 30 is placed between a radiation source 11 and a detector 12 in an examination region 14. The subject 30 is imaged with an (x-ray) radiation beam 13 emitted from the source 11 towards the detector 12. A bow-tie shaped beam shaper 20 is placed between the source 11 and the examining region 14. The beam shaper 20 comprises a radiation absorbing material. The radiation beam 13 is filtered such that photons in the center encounter less radiation absorbing material than photons on the sides. Subject 30 is shaped such that it is thicker at the center and thinner on the sides (as is usually the case for a human body). The center of the subject 30 receives a higher radiation dose than the sides due to bow-tie shape of the beam shaper 20 to account for the additional mass present at the center compared to the sides. The situation shown in FIG. 2a depicts a first extreme situation: the subject substantially covers the entire radiation beam 13. Individual beams 131, 132 and 133 are all (partly) absorbed by the subject 30. Because of this and because the outer beams 131 and 133 are filtered more strongly by the beam shaper 20 and encounter less mass of the subject than the center beam 132 a homogeneous intensity profile is achieved at the detector.
This is an ideal situation in case the subject is symmetric at all irradiation angles. However, in 3D imaging of a non-symmetric subject (such as shown in FIGS. 2a and 2b or in case of a human being), a non-ideal situation occurs. FIG. 2b depicts a second extreme situation: the radiation source 11 rotated to a position where the radiation beam 13 irradiates the subject 30 on its narrowest side. The subject 30 now does not cover the entire radiation beam. The center beam 132 is still fully attenuated by the subject 30, but a path through the subject 30 is much longer than in the situation of FIG. 2a, causing more attenuation and less radiation received by the detector 12, potentially resulting in decreased image quality. On the other hand, outer radiation beams 131 and 133 do not encounter the subject 30 at all and reach the detector 12 without any further attenuation after the beam shaper 20. The intensity profile at the detector is strongly out of balance: the sides of the detector 12 are over-illuminated, while the center is potentially under-illuminated. For this position a different beam shaper configuration would be desirable, e.g. one that is thinner in the center and thicker at the edges than the one shown in FIGS. 2a and 2b. 
An optimal beam shaper configuration changes for each position (or at least multiple positions) of the radiation source 11 with respect to the subject 30 and therefore dynamic beam shapers were developed. These can take into account the angular inhomogeneity of the thickness of the subject. However, known dynamic beam shaping devices might not be usable during all operating modes of 3D radiation systems.
First, hard (i.e. high contrast) edges of an x-ray beam shaper lead to high contrast shadows on the projection images which has implications for the reconstruction algorithms. Second, a very high reproducibility of the attenuation profile is necessary in order to allow for a 3D (tomographic) reconstruction and consistent image quality, which is very challenging for known dynamic beam shapers. Third, depending on the position, the x-ray beam has to be attenuated to as low as 10%-20% of the original intensity. The amount of scattered radiation which is generated due to this absorption process can be significant and strongly depends on the actual choice of the filter material. Scattered x-ray radiation generally decorates the contrast of the projection images. And fourth, dynamic beam shapers need to be able to vary the x-ray beam profile fast enough to account for the fast rotation of state of the art 3D imaging systems. With known dynamic beam shapers this has proven to be rather challenging in combination with the previously stated other issues.
Several dynamic beam shapers for 3D x-ray imagers have been proposed, for instance in DE 10 2012 2217616 A1 or DE 10 2012 223748 A1. In these disclosures a dynamic beam filter is based on pumping liquid into the filter. However, these and other known dynamic beam shapers, are limited by the abovementioned technical issues to different extent, especially matching attenuation with the rotation speed of the source. Also, leakage of the liquid may cause serious damage to the electronics or mechanics of underlying equipment or parts. Because of these reasons these known dynamic beam shapers have not yet found significant clinical and commercial acceptance.